Device for characterizing a sample

ABSTRACT

The present invention relates to a device for optical characterisation of a sample and/or of the material(s) of the same having an illumination unit that can be orientated to illuminate with incident light a sample spatial portion into which the sample can be introduced, a detection unit which is orientated or can be orientated to image the sample introduced into the sample spatial portion by receiving light reflected by the sample, and which is configured to detect at least two different, preferably orthogonal, polarization components in the reflected light, and an evaluation unit with which, in the imaging data recorded by the detection unit, those imaged surface elements (reflection elements) of the sample can be identified, and with which the detected different polarization components for these reflection elements can be evaluated for optical characterisation.

PRIORITY CLAIM TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority of U.S. patent application Ser. No. 14/837,012, filed Aug. 27,2015, which is a divisional application and claims the benefit ofpriority of U.S. patent application Ser. No. 13/818,188, filed May 6,2013, which is a national stage application under 35 U.S.C. §371 ofPCT/EP2011/004553, filed Sep. 9, 2011, and published as WO 2012/038036A1 on Mar. 29, 2012, which claims priority to German Application No. 102010 046 438.4, filed Sep. 24, 2010, which applications and publicationare incorporated by reference as if reproduced herein and made a parthereof in their entirety, and the benefit of priority of each of whichis claimed herein.

TECHNICAL FIELD

The present invention relates to a device and a method for opticalcharacterisation of a sample and/or of the material (or the materials)of the same. The characterisation is thereby effected on the basis ofevaluation of the polarisation of light which is radiated onto thesample and reflected by the sample. The device and the method can beused in particular for surface inspection or also for sorting bulkmaterial by evaluation of the polarisation of the reflected light.

BACKGROUND

Methods for optical characterisation of samples based on reflectometryor ellipsometry are already known from the state of the art. See forexample Thomas Geiler “Polarisationsbildgebung in der industriellenQualitätskontrolle” (Polarisation imaging in industrial qualitycontrol), VDM Press, August 2008. Devices for classification of samplesin the form of bulk material are also known from the state of the art(WO 2009/049594 A) which operate on the basis of light which isreflected on a retroreflector, which can be polarisation-selective, anddetected with respect to its various polarisation components.

However, all these implementations demand either a planar surface of thetest pieces or a time-consuming, simultaneous variation of the angle ofincidence and of reflection is implemented. In addition, also avariation in the polarisation of the illumination is often implemented.

SUMMARY

It is the object of the present invention to develop devices and methodsfor optical characterisation of samples, in particular devices (andcorresponding methods) based on the technology of reflectometry orellipsometry and polarimetry respectively such that the samples (inparticular also non-planar samples or test pieces, e.g. in the form ofbulk materials) can be characterised easily and reliably with respect totheir material/materials. It is also the object in particular toconfigure the devices and the corresponding methods such that thischaracterisation can be effected rapidly (in the range of a few 1/10seconds up to a few seconds), i.e. in step with the production ofsamples (in-line) or with a bulk material flow.

This object is achieved by a device according to claim 1, by a deviceaccording to claim 12 and by a method according to claim 14.Advantageous embodiments of the devices according to the invention andof the method according to the invention can be deduced respectivelyfrom the dependent patent claims.

In the following, the present invention is described firstly in general,then in detail with reference to various embodiments. The individualfeatures of the invention which are described in the embodiments andproduced in combination with each other need not thereby be producedprecisely in the feature combination shown in the respective embodimentwithin the scope of the invention, but rather can also be produced indifferent combinations with each other. In particular, some of theillustrated features can also be omitted or combined in different wayswith further illustrated individual features of the embodiments.

The present invention, as described subsequently, uses as basis thetechnologies of reflectometry or of ellipsometry which are known to theperson skilled in the art. The corresponding bases are known for examplein H. G. Tompkins. W. A. McGahan “Spectroscopic Ellipsometry andReflectometry”, Wiley Interscience, 1999 or in M. Faupel “AbbildendeEllipsometrie und ihre Anwendung” (Imaging ellipsometry and applicationthereof), VDI reports no. 1996: Optische Messung technischer Oberflächenin der Praxis (Optical measurement of technical surfaces in practice);2007 and are therefore not described in detail in the following.

A basic idea of the present invention is based on identifying, for thosesurface elements of the sample which reflect light radiated onto thesample into a detection unit configured for detecting the light, and ondetecting and evaluating, for these surface elements of the sample(subsequently also termed reflection elements), different polarisationcomponents of the reflected light. The sample is illuminated for thispurpose with preferably monochromatic light. (Monochromatic light is notabsolutely necessary but in general is better suited for example in thecase of samples with dispersion). Alternatively hereto or in combinationtherewith, it is likewise possible according to the invention to usemonochromatic, coherent radiation (laser light) for illumination of thesample and, by means of suitable configuration of the detection unitwhich receives the beam components reflected on the sample, to calculateall four Stokes' parameters and to use them for characterisation of thelight reflected on the sample (and hence for characterisation of thesample itself). In every case, at least two different (preferably:orthogonal) polarisation components in the reflected light are hencedetected. (Alternatively to the term of polarisation component, also theterm of “polarisation state” is used subsequently within the scope ofthe invention although, strictly speaking, light can have merely onepolarisation state; however, it is clear to the person skilled in theart with the help of the description respectively, what is intended.)

Within the scope of the subsequently described invention, there isthereby understood by the term of light reflected by the sample, allthat light emanating from the sample, which is received finally by meansof the device according to the invention and can be used for evaluation.The reflected light hence generally concerns the sum of different lightcomponents, namely in particular light components scattered on thesample, light components reflected diffusely on the sample and lightcomponents reflected reflectively on the sample. (Reflected lighttherefore relates precisely to those light components which reach thedetection unit and not to those light components which arrive back atthe illumination unit.) Subsequently, the light reaching the detectionunit due to a reflective reflection is termed also reflected light inshort: this light hence relates to light of those surface elements ofthe sample which reflect the light radiated onto the sample into thedetection unit whilst fulfilling the reflection condition.

A device according to the invention for optical characterisation of asample (or of one or more materials of the same) can hence comprise thefollowing elements: firstly an illumination unit which is orientated toilluminate the sample (the illumination unit can also be directedtowards a spatial portion of the sample or a spatial volume into whichthe sample is introduced for illumination). This device comprises inaddition a detection unit which is configured to detect a plurality ofdifferent (preferably: orthogonal) polarisation components. Thedetection unit is orientated such that light components, reflected bythe sample, of the light radiated onto the sample can be detected.Finally, this device according to the invention comprises an evaluationunit. This can be produced for example as a computer program in apersonal computer. However, it is likewise conceivable to configure theevaluation unit as part of the detection unit (e.g. as evaluationprogram integrated in a camera). In the imaging data recorded by thedetection unit (this device according to the invention is henceconfigured for planar optical imaging of the sample or of a sampleportion), those imaged surface elements of the sample, the reflectedlight of which, received in the detection unit, is based on a reflectionof the incident light on the sample, can be identified with thisevaluation unit. These surface elements of the sample are subsequentlyalso termed reflection elements, in contrast to those surface elementsof the sample which, because of physical effects other than a reflection(e.g. i.e. by light scattering) reflect light into the aperture of thedetection unit. The evaluation unit of this device according to theinvention is finally configured such that the detected differentpolarisation components can be evaluated precisely for the reflectionelements in order to obtain the desired optical characterisation of thesample. On the basis of this evaluation, it is then possible for exampleto separate, with this device according to the invention, objects orobject regions which have a narrowly tolerated range of optical materialconstants from objects with deviating optical material constants(sorting device) or to examine the maintenance of optical materialconstants automatically, e.g. in one production process. Sorting of bulkmaterial is possible in particular with the device according to theinvention.

An essential feature of the above-described solution according to theinvention (device or also method for optical characterisation using sucha device) is hence that, provided it is desired, e.g. by using anevaluation unit with a corresponding computing capacity, also in stepwith a production or even with a bulk material flow, a plurality ofdifferent (e.g. two orthogonal) polarisation components in the lightreflected by the sample can be determined for precisely those surfaceelements of the sample, the normal of which bisects the angle betweenillumination unit and detection unit, and which therefore represent thereflection elements of the surface of the sample. Automatic detection,as to whether a specific surface element (i.e. an observed object point)is a reflection element, i.e. fulfils the above-mentioned reflectioncondition for orientation of its surface normal, can be effected, asdescribed subsequently in more detail, for example by an intensity testof the intensity which is reflected or detected in total by the imagedsurface element.

Within the scope of the present invention, an individual physical objectis not necessarily understood by a sample, a sample can quite generallyalso concern a flow of many individual, moving objects of differentmaterials (i.e. a sample flow within the scope of a bulk material to besorted or to be characterised, i.e. a bulk material flow). As describedsubsequently in more detail with reference to concrete examples, anillumination unit used within the scope of the invention need notnecessarily concern an individual light source, rather also in parallela plurality of suitably disposed light sources which illuminate one andthe same sample can be used. In general, the reflection condition isthen fulfilled for each of the light sources used. Within the scope ofthe present invention, there is understood by reflected light, all thoselight components which originate in light components, radiated by theillumination unit and incident on the sample, and which are not absorbedby the sample, but which again leave the sample or the surfacethereof—generally in a different direction from the direction ofincidence—by means of any processes (reflection, scattering, . . . ) andhence can be detected outside the sample.

An essential idea of the above-described device according to theinvention is hence to detect and evaluate only the beam componentsreflected on the sample (reflectively), i.e. to evaluate only differentpolarisation states for the thus reflected beam components forcharacterisation of the sample, but not for the other reflected, e.g.scattered, beam components. As described subsequently in more detail, itis thereby particularly advantageous to dispose the illumination unitand the detection unit at the Brewster angle provided that thecharacterisation task, when using the device according to the invention,resides precisely in detecting the presence of a defined material or inidentifying individual elements of this defined material in a samplecomprising a large number of objects of different materials (theBrewster angle adjusted in the device is then the Brewster angle of thismaterial). This arrangement at the Brewster angle is advantageous inparticular for the reason that the beam components reflected on samplesor sample elements of this material have merely one polarisationdirection so that the material characterisation is possible in aparticularly simple manner. An arrangement at the Brewster angle ishowever not absolutely necessary since the different polarisationcomponents for the reflection elements on the surface of the sample canalso be evaluated without maintaining this special reflection condition(e.g. a differentiation can be made with respect to the incidentintensities thereof).

In a first advantageous embodiment of the above-described deviceaccording to the invention, the evaluation unit is configured such thatfirstly the reflection elements can be identified in the recordedimaging data before the detected different polarisation components canbe evaluated for these (or based on these) identified reflectionelements. For example, the reflection elements can be established withreference to an evaluation of the total detected intensities of theindividual surface elements (or of the image pixels of the imagesdetected by the detection unit) by those surface elements beingidentified as reflection elements, the total intensity of which (sum ofthe intensities of all the detected polarisation components) lies abovea fixed threshold value (e.g. that intensity value, above which theintensity of 20% of all the imaged surface elements lies, can be definedas threshold value). Then the different (e.g. orthogonal) polarisationcomponents are evaluated merely for the thus identified reflectionelements, e.g. viewed separately or viewed with respect to the intensityratios thereof.

As an alternative thereto, it is also possible, in the recorded imagingdata for all imaged surface elements (these comprising both imagedsurface elements of the sample and imaged surface elements of structureswhich do not belong to the sample but are nevertheless imaged), toconsider firstly the detected different polarisation components, e.g.separately or according to the ratios thereof, and (for example bysetting a threshold value) to evaluate them in order to determine thosesurface elements of the sample which are reflection elements. Thus forexample all surface elements, the intensity of which exceeds apredetermined threshold value for a defined polarisation component inthe imaging data, can be defined as reflection elements. For thereflection elements thus identified by means of the differentpolarisation components, the different polarisation components are thenevaluated further (for example by forming a ratio of the intensities inimaging data or polarisation partial images which correspond todifferent polarisation components and are recorded by the detectionunit) in order to implement the optical characterisation of the sample.

In an advantageous embodiment, the device according to the invention isconfigured such that the differentiation, required for identifying thereflection elements, of reflection elements and of surface elements, thereflected, received light of which is not based on a reflection of theincident light on the sample (scattered elements), is effected on thebasis of the intensity or of the intensities of imaging data of one, ofa plurality or of all of the detected polarisation components. Inparticular, intensity differences or also intensity ratios of thedifferent detected polarisation components can be used for determinationof the reflection elements. It is also possible to use the totalintensity of all polarisation components, detected by the detectionunit, of the light arriving for imaging for identification of thereflection elements. Thus for example, those surface elements, theassociated imaging values of which, in the images detected by thedetection unit, are in total above a predefined threshold value, can beidentified as reflection elements. Such a threshold value can be definedfor example as 90%/10% threshold value, i.e. it can by means of this beestablished that 90% of the total intensity values of all imaged surfaceelements are below this threshold value and 10% above.

As an alternative thereto or also in combination with theintensity-based identification of the reflection elements, thereflection elements can also by defined on the basis of the positionthereof in images of the sample produced corresponding to the differentpolarisation components (or also corresponding to the received totalintensity): for this purpose, the position of the surface elements (e.g.taking into account the intensities thereof) can be evaluated relativeto each other and/or relative to one or more reference point(s) in theimages of the sample. In particular centres or edge points of images ofthe sample can serve as reference points. For example, by suitableconfiguration of the sample background or by additional illumination(which ensures a constant, low background intensity), individualelements of the sample (e.g. bulk material particles) can bedifferentiated from the background by for example the change inintensity at the edge of these sample elements being detected(evaluation of gradients in the image). As an additional condition, thatan observed surface element is a reflection element of the sample, itcan then be established—in addition to the above-described thresholdvalue setting—that the reflection elements must be located within thethus detected outlines of individual sample elements.

In contrast thereto, those surface elements, the intensity or brightnessof which lies below the above-described, adjustable threshold, are thennot reflection elements but scattered elements. Also surface elementswhich are situated outside the sample element limits, which can beidentified as described above, are not reflection elements of thesample. Further evaluation of these surface elements is therefore notsensible.

In a further advantageous embodiment of the above-described deviceaccording to the invention, evaluation of the identified reflectionelements is effected for the purpose of optical characterisation of thesample as follows: a ratio is formed from different polarisationcomponents, which are recorded by the detection unit for the identifiedreflection elements (e.g. from two linear polarisation components whichare orthogonal to each other). This can take place for example by theintensity value, for all the surface elements which were identified asreflection elements, in the image recorded for a first polarisationcomponent, being divided by the intensity value of the correspondingreflection element in the image recorded for a second, different (e.g.orthogonal) polarisation component.

If the thus formed ratio then exceeds or falls below a specific valuefor a specific minimum number of reflection elements (relative to thetotal number of surface elements and/or of reflection elements), theninformation about the presence or absence of a defined material in thesample can hence be obtained: if for example, as reflection conditionfor the reflection elements, the angle between the optical axis of thedetection unit, on the one hand, and the optical axis of theillumination unit, on the other hand, in the triangle which is spannedby the detection unit, the illumination unit and the sample, is adjustedto twice the Brewster angle of a sought material (the reflectionelements are then those surface elements of the sample, the normal ofwhich bisects the angle between the two above-mentioned axes), then allthose reflection elements which can be assigned to the sought materialreflect light components which are polarised merely parallel to thesurface of the sample but not light components with a polarisationdirection perpendicular thereto. However, this can be detected viasetting a corresponding threshold value for the ratio calculated asdescribed above, so that a differentiation of sample elements of thesought material from sample elements made of a different material ispossible.

According to the invention, it is hence possible to test whether theratio of intensities of different polarisation components is within acertain range in order to differentiate defined materials from othermaterials. In particular in the case of bulk material flows as samples,also the absolute number of those image points or surface elements inthe imaging data, which are recorded by the detection unit and for whichthe ratio calculated as described above exceeds or fall below athreshold value, can thereby be used as sorting criterion. Alternativelythereto, it is possible to evaluate not the absolute number of suchsurface elements but the relative number of these surface elements incomparison with those surface elements which do not fulfil the thresholdvalue criterion.

In the above-described embodiment variants of the invention, theconsideration is crucial that, even with irregular surfaces of samples(e.g. of bulk materials), there is at least one point in the case ofeach object or element of the sample, i.e. a surface element, whichfulfils the reflection condition and with which the object can hence becharacterised.

It is particularly advantageous, within the scope of the invention, tomake use of surface elements or reflection elements from differentdirections at the same time or also in succession for the evaluation.Thus of course it basically suffices that the illumination unit used hasmerely a single illumination element (e.g. a single monochromatic lightsource, see subsequent embodiment 1).

However, the illumination unit can also comprise a plurality ofindividual illumination elements which are configured to illuminate thesample with incident light from different directions. The angle betweenthe detection unit or the optical axis thereof, on the one hand, and therespective illumination element or the optical axis thereof, on theother hand, in the triangle which is spanned by the detection unit, thecorresponding illumination element and the sample can thereby beidentical in all illumination elements. Advantageously, two or fourindividual illumination elements can be used. The illumination elementscan be disposed on the side of the sample situated opposite thedetection unit and in a plane orientated preferably perpendicular to theoptical axis of the detection unit.

In particular, the individual illumination elements can be disposed atequidistant angle spacings on a circle about the optical axis of thedetection unit in this plane. For example, when using four illuminationelements, these can thus be disposed at angle spacings of 90° on acircle about the optical axis of the detection unit. For all theseillumination elements, the angle ratios described above for theillumination unit (e.g. adjustment to a Brewster angle for a definedmaterial) can then be maintained.

All the devices for optical characterisation described within the scopeof the present invention can be configured by suitable provision offurther components (e.g. sample storage units etc.) for surface testingof planar coatings as sample.

However it is likewise possible to develop the devices forcharacterisation, differentiation and/or separation of individualelements of a sample comprising a large number of elements (inparticular: bulk material flow). This can take place for example by theillumination unit and the detection unit being disposed for illuminationand imaging of a free falling stretch part, e.g. below a vibrator forbulk material. As an alternative thereto, of course, also conveyer beltportions on which bulk material is transported can be illuminated by theillumination unit and scanned by the detection unit. The last-mentioneddevices can then be configured in particular also for sorting sampleelements which deviate from one or more predefined material parameter(s)(which is/are determined for optical characterisation by evaluation ofthe reflection elements).

The above-described devices according to the invention can be configuredas a laser scanner system with an illumination unit which scans one- ortwo-dimensionally the sample or sample spatial portion in which thissample is disposed, on the basis of one or more laser(s) and with one ormore suitable receiving unit(s) as detection unit.

As a alternative thereto, it is however also possible to use one or moremonochromatic light sources as illumination unit or illuminationelement(s). As detection unit, one or more camera(s), in particularpolarisation camera(s) and/or CCD-based camera(s), can then be used.

Illumination of the sample is effected advantageously with one or moredefined wavelength(s) in the visible range; it is however basically alsoconceivable to use for example infrared radiation for the illuminationprovided that the receiving units are then correspondingly adapted.

Subsequently, a few concrete embodiments of the illuminationunit-detection unit system of the present invention are now described:

Thus, a camera which is used in a reflection arrangement, as describedabove, and with which for example two orthogonal polarisation componentscan be detected for example for each scanned surface element, can be acamera consisting of two individual cameras. In the beam path in frontof the individual cameras, a polarising optical element (e.g. prism orbeam splitter) is disposed, with which the light reflected by the samplecan be split into two different polarisation components. The light ofthe one polarisation component is then directed by the polarisingoptical element towards the one individual camera, the light of theother polarisation component towards the other of the two individualcameras. It is thereby advantageous to provide a pixel adaptation inorder to coordinate the position of the detected reflection elements inthe images of both individual cameras.

As an alternative thereto, a multiline camera can be provided, a numberof polarisers which corresponds to the number of lines of the camerabeing provided in the beam path in front of this camera. Two (or more,e.g. six) different types of polarisers, for example two polarisersorientated orthogonally relative to each other (in the case of sixpolarisers, e.g. 0°, 45°, 90°, 135°, left-circular and right-circularpolarising polarisations) thereby exist. The two or more polarisationdirections are detected by the camera: in front of the individual linesof the camera, polarisers of the one type and of the other type, in thecase of two different polarisers, are disposed alternately so that lightof a first polarisation component and of a second polarisationcomponent, e.g. orthogonal to the first polarisation component, isimaged respectively alternately onto the camera lines. In the case offor example six different polarisers, accordingly six adjacent lines forthe six different types are required.

As an alternative thereto, a camera which, in the beam path in front ofits sensor chip, comprises a polarisation strip filter or a polarisationmosaic filter can be used. Such a filter splits the light reflected bythe sample into the different polarisation components which then aredirected, in lines or corresponding to the mosaic arrangement of thefilter, towards the respective sensor cells of the camera chip (whichsensor cells of the chip receive light of which polarisation componentis known on the basis of the already known filter shape so that theevaluation can be correspondingly effected).

It is likewise conceivable to provide a plurality of differentlypolarised illumination sub-units (e.g. individual lamps) which are allorientated to illuminate the sample with incident light. The individualillumination sub-units are switched on and off again temporally insuccession, therefore illuminate the sample respectively in successionfor a predefined time duration. The individual polarisation componentsare then detected during different, precisely defined time intervalsrespectively by the total camera surface of a non-polarisation-sensitivecamera (preferably a multiline camera is used), hence the differentlypolarised illuminations are quasi-flashed. If the device is used withmoving samples (bulk material flow), the flashing frequency or thefrequency of the switch-over between the individual illuminationsub-units must be synchronised advantageously with the sample speed.

This synchronisation has the following advantage: in general, the sampleis in motion during the scanning (e.g. on a corresponding fallingstretch or also flight stretch in the case of a for example parabolicdischarge from a conveyer belt, i.e. the image of the sample is moved,in the case of the individual camera images taken temporally insuccession for evaluation, within these camera images, i.e. changes itsposition in the individual camera images. Since now however generallyboth (or the more than two) polarisation states are required from oneand the same point or surface element of the sample, it must in generalbe known how far the sample has moved between two adjacent camera imagesin order also to detect and evaluate in fact the polarisation states ofone and the same surface element of the sample (it is thus establishedfor example that the sample is moved between two temporally adjacentcamera images by 5 pixels with respect to its image so that acorresponding displacement of the temporally adjacent camera images canbe effected in order to compensate for the sample movement).

A further camera-based system construction according to the inventionfor the illumination unit and the detection unit uses an LEDillumination in which the individual LED illumination elements whichemit light of one wavelength are disposed such that, for each imagepoint or for each imaged surface element, the same reflection conditionis given over the entire sample surface to be scanned (total cameraimage). This can be achieved by different angles of inclination of theindividual LED elements in a strip, by means of an arrangement of theseelements on a bent plate or in the case of a planar arrangement of theLED elements by means of an attachment lens system.

By means of the bent plate or the attachment lens system, in particulareffects of the e.g. funnel-shaped camera opening can be compensated for(which would otherwise prevent exact detection and evaluation): becauseof the corresponding bent plate or attachment lens system, the camerathen no longer has a telecentric beam path but rather a fan-shaped onewhich leads to the fact that actually the same reflection conditions aregiven for each image point over the entire sample surface to be scanned.

Within the scope of the scope of the present invention, it is alsopossible to provide the illumination unit which is used with means forchanging and/or adjusting the polarisation of the illumination (forexample LC element, as used in LC displays).

The illumination of the sample can then be effected, as in the case ofthe “flashing” described earlier, in succession with differentpolarisation states. For each illumination-polarisation state, differentpolarisation components for the reflection elements can then, asdescribed earlier, be detected and evaluated.

As an alternative to switching on and off or illumination of the sampleby “flashing”, also suitable polarisers which produce the desiredpolarisation states can hence be placed in front of the illuminationunit (or in the beam path between illumination unit and sample).

It is also possible to use the illumination unit and the detection unittogether with a polarisation-obtaining retroreflector. A flow diagram ofsuch an illustration is provided in FIG. 6. In the case of a laser usedin the form of a laser scanner as illumination unit and in the case of areceiver configured suitably as associated detection unit (which canhave in particular a polarisation-obtaining beam splitter for dividingthe incident laser light into two partial beam paths and also polarisingoptical elements in these partial beam paths), the illumination unit andthe detection unit can also be configured as integrated transmitting andreceiving unit 11. A retroreflector 13 must then be provided, thecombined transmitting and receiving unit and the retroreflector beingconfigured and disposed as follows: in the combined transmitting andreceiving unit 11, transmitting and receiving beam path are coupled viaa beam splitter 8 on the same axis. The transmitter illuminates thesample P, the beam components which are reflected by the sample P (i.e.scattered, diffusely reflected or reflectively reflected) are reflectedback on the retroreflector 13 per se and arrive at the combinedtransmitting and receiving unit 12, preferably via the sample P, andthere, via the beam splitter 8, at the receiver beam path of thereceiver of this combined transmitting and receiving unit. FIG. 6 is aflow diagram and does not illustrate angles of transmitted or reflectedlight.

With a corresponding arrangement of a laser, which has a non-integratedconfiguration, and of a receiver which is suitably configured andorientated to receive the reflected light of the laser, theretroreflector is disposed in the above-described case wherever thereceiver stands. This has the advantage that if the beams componentsarrive at the combined transmitting and receiving unit via the object,the radiation is reflected twice on the sample or on the object. Henceimproved imaging of the sample or of the object with an improvedintensity ratio is effected (if for example in the case of a singlereflection on the sample, an intensity ratio of 1:1,000 is present, thenthe intensity ratio with this construction is 1:1,000²). By means ofsuitable retroreflectors or adjustment of the back-reflection, inaddition a very compact construction can be produced. Finally, withinthe scope of the devices for optical characterisation as described aboveand according to the invention, it is also possible (and in particularin the above-described retroreflector variant) to use a laser whichscans the sample in lines or in a grid shape as illumination unit (laserscanner system). The detection unit then comprises a receiver, which issuitable for receiving the laser light reflected by the sample, with oneor more optical element(s) for separation of the received laser lightaccording to the different polarisation components. In the receiver, aplurality of receiving elements are configured, the number of whichcorresponds to the number of partial beams resulting from theseparation. The evaluation unit can establish, for example by testingthe received total intensity, whether surface elements are reflectionelements of the sample, i.e. fulfil the reflection condition. Theevaluation unit of the above-described devices for opticalcharacterisation according to the invention is configured such that,with it, the reflection elements of the sample can be identified (forexample by evaluation of the total intensity received by each objectpoint as a criterion for whether the corresponding object point fulfilsthe reflection condition, i.e. has the orientation of its surface whichis required for further evaluation). The reflection elements are thenevaluated further by the evaluation unit for optical characterisation ofthe sample, i.e. one or more processing step(s) is/are provided in orderto implement an overall characterisation of the sample by evaluation ofthe significant surface elements or of the reflection elements. In orderto detect further sample- and/or material parameters of the samplewithin the scope of this overall characterisation, in addition, delayelements (e.g. λ/4 plates) can be provided in front of the illumination.It is also possible to use further beam splitters or filters in front ofthe detection unit or the light-sensitive surface thereof. Theorientation, adjustment and arrangement of such delay elements and/orbeam splitters or filters can be effected such that determination offurther Stokes' parameters is possible. In particular also amonochromatic coherent illumination unit (laser) can be provided sothat, due to physical secondary conditions known to the person skilledin the art, merely three Stokes' parameters are required instead of fourStokes' parameters for complete characterisation of the polarisationstate of the light reflected by the sample.

According to the invention, systems with angles of incidence andreflection between the two special cases 0° and 180° are possible: inthe boundary case of 90°, the sample should hereby be placed betweenillumination unit and detector. In the special case of 90° and whenusing a combined illumination- and detection unit or transmitting andreceiving unit, the retroreflector is situated behind the sample. In thespecial case of a 90° arrangement, this involves a polarimetryconstruction for the optical characterisation of transparent objects. Inthis case also, in comparison with arrangements according to the stateof the art, the advantage is gained that a significantly fasterevaluation can be effected: by means of a complete characterisation ofthe polarisation state, the variation in the illumination conditionsrequired with devices according to the state of the art can be dispensedwith. As a result, a test in step with the production of goods ispossible for the first time. Just as in the case of the reflectionevaluation, a test of the overall intensity is sensible in this case inorder to find the object points or surface elements which are relevantfor characterisation of the sample. However, in this case, these pointsare not reflection elements but transmission elements. In this case, thelight components reflected by the sample can therefore also concerncomponents transmitted through the sample (transmission instead ofreflection or also both components: transmission and reflection). Thedevice can therefore also be configured as transmission system ortransmission-reflection system. In a further embodiment, the deviceaccording to the invention for optical characterisation comprises thefollowing elements: a laser orientated for one- or two-dimensionalscanning with incident light of a sample spatial portion in which thesample can be introduced (illumination unit). For receiving the lightreflected through the sample, a receiver which is suitable for receivinglaser light is provided as detection unit. This receiver comprises afirst, preferably polarisation-obtaining beam splitter for dividing thelaser light incident on the receiver into a first and a second partialbeam path. In each of these two partial beam paths, a polarising opticalelement (e.g. polarising prism or polarising beam splitter) is provided,with which the light of the respective partial beam path can be splitinto two different polarisation components. In the beam path of each ofthe two thus separated polarisation components, respectively onereceiving element is provided, with which the respective polarisationcomponent can be detected (hence in total four receiving elements areprovided, two in each of the above-described partial beam paths). Apolarisation-changing element is provided merely in one of the twopartial beam paths in addition after the beam splitter and in front ofthe polarising optical element, with which polarisation-changing elementthe polarisation of this partial beam path can be changed. This changingelement can concern in particular a delay plate which is configuredpreferably as λ/4 plate. The evaluation unit of the device is configuredsuch that, with it, on the basis of the different polarisationcomponents detected by the plurality of receiving elements of thereceiver, the polarisation state of the light reflected by the samplecan be determined completely for the optical characterisation of thesame.

For this purpose, in particular the beam splitter, the polarisingoptical elements, the changing element and the four receiving elementscan be configured, disposed and adjusted such that three or four Stokes'parameters of the reflected light can be calculated from the detecteddifferent polarisation components. Since the incident laser light iscompletely polarised, determination of three of the four Stokes'parameters suffices to calculate the fourth Stokes' parameter (using asecondary condition for monochromatic, coherent light) and hence tocharacterise completely the polarisation state of the reflected light.With the help of the polarisation state of the reflected light, which isthus determined completely, for example different materials of differentsample elements of a bulk material sample can then be identified anddifferentiated.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described subsequently in detail with referenceto several embodiments.

There are thereby shown:

FIG. 1 a first embodiment of a device according to the invention usingan individual illumination element as illumination unit.

FIG. 2 a further embodiment of the invention in which the illuminationunit consists of two separate illumination elements.

FIG. 3a to 3d examples of identification of a defined material in a bulkmaterial flow of different materials.

FIG. 4 an example of a device according to the invention configured astesting system for coatings.

FIG. 5 a further embodiment of the invention which is configured forcomplete characterisation of the polarisation state of the reflectedlight.

FIG. 6 a diagram of a device with a retroreflector and a transmittingand receiving unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a ) shows a device according to the invention which is configuredfor characterisation of individual sample elements or test pieces P inthe form of a bulk material sorting system. The individual objects ofthe bulk material flow or of the sample P are transported on a planarconveyer belt 30, the outer surface of which, on which the elements ofthe sample P come to be situated, is white. This serves for betteridentification of the individual sample elements in the image (seesubsequently). The conveyer belt 30 is actuated by two rollers 31, 32;transport of the sample elements P is effected here to the right in theimage (arrows); further elements of the bulk material sorting device(e.g. blowing units or collection containers for the sample elements ofdifferent material) are not shown here.

The illumination unit 2 of the illustrated device comprises amonochromatic light source 21 which is configured here as LED strip andemits in the green range (550 nm). A diffuser 22 which reduces themodelling of the LED structure 21 is disposed in the beam path after thelight source 21. In the beam path behind light source 21 and diffuser22, the illumination unit 2 has in addition also a polariser 23. Theoptical axis of the illumination unit 2 consisting of the elements 21,22 and 23 is characterised here with the reference number 2 o. The lightE incident on the sample P along the optical axis 2 o of theillumination unit 2 is incident at an angle θ_(B) (relative to thenormal N to the surface of the conveyer belt 30 covered with theindividual sample elements) onto the surface of the sample spatialportion 1 which here comprises a defined surface segment parallel to thelongitudinal direction of the conveyer belt 30. The correspondingconveyer belt portion is provided here with the reference number 7.

The detection unit 3 of the illustrated system is disposed, relative tothe conveyer belt 30, in the same half-space as the illumination unit 2(i.e. in the half-space situated above the conveyer belt 30) but, viewedrelative to the conveyer belt portion 7 or to the sample spatial portion1 illuminated by the illumination unit 2, is disposed in this half-spaceon the side situated opposite the illumination unit 2. The optical axisof the detection unit 3 configured as polarisation camera is describedhere with the reference number 3 o.

The illumination unit 2 or the optical axis 2 o thereof, the centre ofthe sample spatial portion 1 or of the illuminated conveyer belt portion7 and the detection unit 3 or the optical axis 3 o of the same, form anisosceles triangle, the longitudinal side of which is formed by theconnection line light source 2—detection unit 3 and the cathetus ofwhich is formed by the connection lines light source 2—sample spatialportion 1, 7 and sample spatial portion 1, 7—detection unit 3(reflection arrangement). The normal N of the longitudinal side of thistriangle or the normal to the conveyer belt surface hence bisects theangle between the two optical axes 2 o and 3 o into two angles θ_(B) ofequal size, here θ_(B)=63° applying.

An evaluation unit 4 in the form of a personal computer with suitablyconfigured evaluation programs is connected to the detection unit 3 viaa bidirectional data line.

The mode of operation of the device illustrated in FIG. 1a ) isdescribed subsequently.

The device is adjusted to differentiate sample elements made ofzirconium from sample elements made of glass. For this purpose, theangle θ_(B)=63° was adjusted to the Brewster angle of the materialzirconium. The evaluation or the optical characterisation is now basedon the idea that the surface portions of the individual sample elements,which are orientated towards the illumination unit-detection unithalf-space, can constantly be differentiated, that there is hence (cf.FIG. 1b ) at least one surface element for each sample element P, thenormal of which surface element is orientated parallel to the normal Nor to the angle bisector of the two optical axes 2 o, 3 o. For such asurface element of a sample element P, the incident radiation E henceimpinges on the surface of the sample element P precisely at theBrewster angle θ_(B) of zirconium.

FIG. 1b ) illustrates how those surface elements, for which thisreflection condition is fulfilled and which are therefore reflectionelements 5 of the sample elements P, can be differentiated from otherimaged surface elements of the sample or from imaged surface elements ofthe background or of the conveyer belt surface (these surface elementsare subsequently described in summary as scattered elements 6 althoughthe physical process underlying their imaging can also be a processother than a scattering process): if in fact (relative to the overallreflected light radiation Z) not only does a reflected beam component Z1arrive at the detection unit 3 from reflection elements 5 of the sampleP and lead there to imaging of the corresponding surface element by thedetection unit 3, but also light Z2 which is scattered for example on ascattering element 6 likewise arrives at the detector 3 (in FIG. 1b ),this is for example light which has been reflected already once on thesurface of the conveyer belt 30 and is therefore incident, from adirection of incidence E2 which does not coincide with the optical axis2 o, onto the scattering element 6 of the surface of the sample P, whichlight is then scattered in the direction Z2=Z1 into the polarisationcamera 3). However scattered light from scattering elements 6 can bedifferentiated from reflected light from reflection elements 5 byevaluation of the intensity of a polarisation component recorded by thepolarisation camera 3 (see subsequently) or also by evaluation of theincident overall intensities of all detected polarisation components.Thus the reflection elements 5 effect for example a significantly higheroverall intensity which impinges on the corresponding image element ofthe polarisation camera 3 than the scattering elements 6. The two types5, 6 of surface elements can therefore be differentiated by setting apredetermined threshold value (which can be determined for example froman average intensity over the entire image). Surface elements 5 whichfulfil the reflection condition are therefore particularly bright in theimaging. These surface elements 5 alone are then evaluated further forcharacterisation of the sample P or the individual sample elementsthereof.

In order to ensure that the specific reflection elements 5 in fact alsoconcern imaged surface elements of sample elements P (and not forexample light components reflected on the white background or on thesurface of the conveyer belt 30), in addition the position of thepotential candidates for reflection elements 5 can be evaluated in thetotal recorded image: by means of image processing algorithms for edgedetection, known to the person skilled in the art (search for closedcurves in the image which is differentiated once or twice and thresholdvalue-treated), the position, the size and the shape of the individualsample elements of the sample P can be established for example.Reflection elements R can then be merely those surface elements orpoints in the image which come to be situated inside the image of asample element or inside such closed curves. In order to determine thereflection elements 5, a combination of intensity- and positionevaluations can therefore be used (only particularly bright surfaceelements in the central region of the imaging of a bulk material objectP can hence be reflection elements 5 in the system of FIG. 1).

Further evaluation of the identified reflection elements 5 in the imageof the camera 3 and the sample material characterisation based thereonthen takes place as follows: the polarisation camera 3 is configured forseparation of two orthogonal polarisation components, namely of thepolarisation component of light E which is incident parallel to theplane of incidence of the reflection elements 5 (plane parallel to theconveyer belt surface) and of the polarisation component incidentperpendicular thereto. If an imaged sample element P concerns an elementmade of zirconium, then, since here the Brewster condition is fulfilled,merely light polarised parallel to the above-described plane isreflected. Only this polarisation component can hence be detected forzirconium sample elements P with one channel of the camera 3, whilst theother channel of the camera 3 (which is configured for detecting lightpolarised perpendicular thereto) can detect no reflected light. If theobserved sample element P concerns an element made of a material otherthan zirconium, then light of both polarisation components is detectedby the polarisation camera 3 (i.e. both channels of the camera areaffected). If the ratio of the intensities of both polarisationcomponents in both channels or images of the sample spatial portion 1,recorded by the polarisation camera 3, is hence formed for all thosesurface elements which are reflection elements 5, then this ratio variessignificantly for reflection elements of zirconium surfaces and forreflection elements of surfaces of other materials. By setting asuitable threshold value, zirconium sample elements can hence bedifferentiated from other sample elements.

If light which is polarised for example parallel to the plane ofincidence is displayed by the polarisation camera in blue and lightpolarised perpendicular thereto in red, then this means that, in theimages recorded and superimposed by the polarisation camera, thereflection elements of zirconium sample elements P appear to be purelyblue.

FIGS. 3a to 3d show examples of the differentiation of diamond andquartz glass (FIG. 3a to 3c ) and of zirconium crystals in a bulkmaterial flow P of such crystals, of glass shards and of metal rings(FIG. 3d ). The polariser 23 was adjusted for FIG. 3d such that theconveyer belt surface (background) reflects both polarisation directionswith the same intensities in the direction of the polarisation camera 3.θ_(B) is thereby 63° (Brewster angle for zirconium). Even if the sampleelements P conveyed through the sample spatial portion 1 have anirregular geometry and their precise position is unknown, neverthelessan object characterisation is possible since the individual sampleelements have differentiatable surfaces, i.e. each bulk materialparticle has at least one surface element which fulfils the reflectioncondition (angle of incidence=θ_(B)). Since reflection provides verymuch stronger signals than scattering, these surface elements can beidentified. The underlying physical principles of this reflectometry(reflection of polarised light on the medium, Fresnel formulae forperpendicular and for parallel polarisation and also the law ofrefraction) are known to the person skilled in the art.

FIG. 3a shows how a sorting criterion can be developed from the Fresnelformulae by calculation of curves for the relative reflection capacityof two different materials: FIG. 3a shows the reflection capacity as afunction of the angle of incidence for the materials quartz (refractiveindex=1.46) and diamond (refractive index=2.41), i.e. in the case wheredifferentiation of diamond from quartz glass is desired. The Figureclearly shows the different Brewster angles for the two materials; forseparation of the two materials, an arrangement at the Brewster angleθ_(B) of the sought material can hence be effected (i.e. for diamond atθ_(B)=67.5°). Those reflection elements 5 in which merely onepolarisation component remains after reflection can then be determined.R_(S) is the reflection capacity for light polarised perpendicular tothe plane of incidence and R_(P) is the reflection capacity for lightpolarised parallel to the plane of incidence.

An increase in sensitivity for separation of the two materials can beeffected by adaptation of the illumination. For example in anarrangement for differentiating zirconium (θ_(B)=63°), the illuminationcan be adjusted by means of the polariser 23 such that the two reflectedintensities are the same for the extraneous material (for example glassor metal). This adjustment can be effected by means of the polariser 23such that then an extraneous material sample is brought into themeasuring field and subsequently the position of the polariser ischanged thus until both intensities are the same.

FIG. 3b shows again, for the example of diamond/quartz glass, the degreeof reflection, likewise (cf. FIG. 3a ) as a function of the angle ofincidence θ.

In the case of an arrangement in which the Brewster angle θ_(B) ofdiamond is chosen as angle of incidence, the characteristic line shownin FIG. 3c is finally produced for the optical differentiation ofdiamond and of materials with a deviating refractive index (e.g. quartzglass). For the illustrated example, the polarisation of theillumination was adjusted such that not reflecting, but scatteringparticles or surface elements with R_(P)=R_(S) have a quotient of 5.Hence a sorting criterion which falls monotonically within a wide rangewith the refractive index n is produced.

When adjusting the system for identification of zirconium, the ratiobetween blue and red channel is then highest on the surface elements 5of the zirconium crystals which are orientated parallel to the conveyerbelt surface and fulfil the reflection condition. The ratio can hence beused for the purpose of identifying the zirconium crystals in theindividual sample elements of the bulk material flow.

FIG. 3d shows a corresponding result, in which, for formation of theratio, the blue channel B has been divided by the sum of both channelsR+B (R=red channel intensity). After setting the threshold value (FIG.3d on the right), it can be readily detected that zirconium crystals aremarked in image 3. The glass shards (further irregular elements in FIG.3d on the left) and a metal ring present in the bulk material flow (FIG.3d on the left at the top) remain dark, i.e. are not identified.

FIG. 4 illustrates a further test task which can be achieved with thedevice according to the invention shown in FIG. 1: paper/gauze is coatedwith Vaseline during production. What is sought is a test system whichautomatically tests the entire coating during production. The approachfor the solution according to FIG. 1 is based here on reflectometry, thelaminate sample P which is flat here being disposed at the Brewsterangle θ_(B) of 55.5° for Vaseline. The curves calculated from theFresnel formulae for the reflection capacity of Vaseline are shown inFIG. 4. By way of comparison, the expected course of a paper scatteringhomogeneously with 20% is plotted. Here also, the separation of the twomentioned materials can be implemented again by evaluation of the twochannels of the polarisation camera 3 (test on R_(S)=0, i.e. for thepresence only of reflected light polarised parallel to the interface).The result of the test is the information as to whether paper is coatedwith Vaseline or not. Paper surfaces coated with Vaseline are hencedistinguished by an intensely blue colour, merely the blue channel ofthe polarisation camera 3 responds. The degree of polarisation of theimaged surface elements can hence be calculated from the intensities Bof the blue channel and from the intensities R of the red channel asfollows: B/(B+R). Vaseline-coated surface components hence produce thevalue B/(B+R)=1. For production control, for example the coated surfacecomponent on the total surface component can be evaluated.

FIG. 2 shows a further device according to the invention in which aplurality of individual illumination elements 2 a, 2 b, as illuminationunit 2, are used in the form of monochromatic light sources withemission wavelengths of respectively λ=550 nm. Viewed in the directionof incidence, a diffuser 22 a, 22 b and a polariser 23 a, 23 b aredisposed behind each illumination element 2 a, 2 b, similarly as shownin FIG. 1. The detection unit 3 and the evaluation unit 4 (not shownhere) are configured similarly to the case described in FIG. 1(differences see below). The illustrated device is configured as bulkmaterial sorting device in which the bulk material (of which only asingle sample element P is shown here) traverses a sample spatialportion 1 in the form of a free falling stretch part 6 f below avibrator (not shown). The optical axis 3 o of the polarisation camera 3is situated here in a horizontal plane perpendicular to the fallingdirection F of the sample elements P. In each of the half-spacesconfigured on both sides of this horizontal plane, an illuminationelement 2 a, 2 b (together with associated diffuser 22 a, 22 b andpolariser 23 a, 23 b) is disposed respectively. The angle between thetwo optical axes 2 oa and 2 ob of the two illumination elements 2 a, 2 band of the above-described horizontal plane is respectively the same,the illumination elements 2 a, 2 b and the camera 3 are thereby disposedsuch that their optical axes 2 oa, 2 ob and 3 o are situated in a planeperpendicular to the above-described horizontal plane.

Due to this arrangement, the reflection condition for the twoillumination elements respectively is hence the same: the angle bisectorN_(a) divides the angle spanned by the two optical axes 2 oa and 3 o orthe angle spanned by the direction of incidence E_(a) of the upperillumination element 2 a and of the reflection direction Z1 into twoangles θ_(aB) of equal size, which are configured corresponding to theBrewster angle θ_(B) of a material to be identified in the sample flowP. The angle bisector N_(b) likewise divides the angle spanned by theoptical axis 2 ob of the lower illumination element 2 b (i.e. theincident light E_(b)) and the optical axis 3 o of the polarisationcamera 3 (or the corresponding reflected imaged light component Z1) intotwo angle portions θ_(bB) of equal size. Due to the above-describedarrangement, there applies here θ_(aB)=θ_(bB). Both illuminationelements 2 a and 2 b are hence adjusted to one and the same angle, theBrewster angle of the material to be identified.

Identification of the reflection elements 5 and the subsequentevaluation of the polarisation components for these reflection elementsfor optical characterisation of the sample elements P is now effectedanalogously to the case described for FIG. 1. However the reflectioncondition for the partial system consisting of the illumination unit 2 aand the camera 3 is fulfilled at a different, later point in time thanfor the further partial system consisting of the illumination element 2b and the camera 3: if a sample element P falls through the illustratedfalling stretch F, then surface elements situated on the rear-sidethereof (in the image: side situated at the top) fulfil the reflectioncondition, i.e. are reflection elements 5 when the observed sampleelement P is disposed, with its rear-side, exactly at the height of thehorizontal plane of the optical axis 3 o, this horizontal plane istherefore precisely the tangential plane to the rear-side of the sampleelement P. For the system 2 b, 3, the reflection condition is incontrast already fulfilled at a point in time preceding this point intime, namely when the front-side of the falling sample element P (theside situated at the bottom in the image) impacts precisely from aboveon the horizontal plane of the optical axis 3 o, this horizontal planeabuts therefore tangentially at the front-side of the sample element P.

The significant surface elements of the sample P which are potentiallypossible as reflection elements 5 must hence be situated in the imagesrecorded currently by the camera 3 o initially on the front-side andthen on the rear-side of the imaged object (which can be identifiedagain by for example gradient-based image processing mechanisms with theaid of its outline). In this respect, the conditions for identificationof the reflection elements 5 differ from those of the system shown inFIG. 1 in which the reflection elements 5 must be situated for instancein the centre of the images of the individual identified sampleelements. Apart from the above-described differences duringidentification of the reflection elements 5, evaluation of the differentpolarisation components for the identified reflection elements canhowever be effected for optical characterisation of the sample Pentirely analogously to the case described for FIG. 1.

Analogously to the case shown in FIG. 2, an illumination unit whichcomprises, instead of two illumination elements 2 a, 2 b, in total fourillumination elements which are disposed in a plane perpendicular to theoptical axis 3 o and equidistantly on a circle about this optical axis 3o (angle spacings of the individual illumination elements 90°) can alsobe used. Similarly to the case shown in FIG. 2, illumination is theneffected such that surface elements on the edge of the objects P areexamined from four directions on the basis of intensity as to whetherthey have matching surface normals N. Hence characterisation of thefalling sample elements P of the bulk material flow with up to fourpoints is possible.

For the objects in FIG. 2, it can hence be tested whether there aresurface elements or points with a pure colour, e.g. blue (cf.description for FIG. 1: then merely one of the two polarisationcomponents is present) and whether these points are situated on thefront- or rear-side of the respective sample elements P (relative to thedirection of movement F).

With corresponding adjustment to the Brewster angle and in the case offour individual illumination elements at a 90° spacing (not shown),objects made of the material to be identified are characterisedaccording to the Brewster angle θ_(aB)=θ_(bB), for example by blue imageelements at a second, later point in time (scanning of the front), redimage elements at a first later point in time (scanning of the left andof the right side) and by further blue image elements at a third, stilllater point in time (scanning of the rear).

FIG. 5 shows finally a further device according to the invention for theoptical characterisation of a planar, laminate sample P in a samplespatial portion 1 on the basis of a laser scanner system. The laser 2 asillumination unit, which scans, one-dimensionally, the sample spatialportion 1 in the direction SR perpendicular to the direction ofincidence E of the light, beams light at the angle of incidence θ (anglebetween the sample normal N and the direction of incidence E of thelaser light) onto the sample surface of the sample P. Cf. in thisrespect FIG. 5 on the right at the top which shows a sectionperpendicularly through the irradiated sample surface and FIG. 5 in thecentre at the right which shows a plan view on the irradiated samplesurface, i.e. a view in the direction of the normal N. The light Zreflected at the corresponding angle of reflection θ (reflection law) isconducted for evaluation to the receiver 3 shown on the left and centrein FIG. 5.

The device shown in FIG. 5 is based on the observation that the emissionlaser 2 of the illustrated scanner emits monochromatic, coherentradiation so that the radiation E received by the sample is alreadycompletely polarised. In this case, the detection of three Stokes'parameters from the reflected laser light components Z hence sufficesfor complete characterisation of the polarisation state of the reflectedor detected light radiation Z.

Viewed in the irradiation direction of the reflected light component Z,the illustrated receiver 3 now comprises in succession in the beam paththe following components:

-   -   A hollow mirror 40 configured for focusing the light component Z        reflected on the sample surface P towards a beam splitter plate        8.    -   The polarisation-obtaining beam splitter plate 8 with which        respectively 50% of the incident, reflected radiation Z is        divided into a first partial beam path T1 and into a second        partial beam path T2.    -   In the first partial beam path T1: firstly a delay plate (λ/4        plate) 9 which directs the light of the first partial beam path        T1 towards a first polarisation beam splitter 10 a which is        configured for differentiating two polarisation components of        the incident light which are orthogonal relative to each other.        The first of these two polarisation components is detected with        a first receiving element 11 a, the other of these two        polarisation components with a further receiving element 11 b        (intensity detectors).    -   The second partial beam path T2 is basically constructed just        like the first partial beam path T1, however the delay plate 9        is omitted here so that, in this partial beam path, merely a        second polarisation beam splitter 10 b and two further receiving        elements 11 c and 11 d are disposed, with which the two        polarisation components which are orthogonal relative to each        other can be detected in the second partial beam path T2.    -   The four receiving elements 11 a to 11 d are then connected        respectively via bidirectional signal lines to an evaluation        unit 4 (not shown).

With the illustrated receiver 3, the polarisation state of the reflectedradiation Z can hence be characterised completely as follows:

With the help of the receiving elements 11 c and 11 d of the partialbeam path T2, the intensities I₀ and I₉₀ for two linear polarisationcomponents which are orthogonal relative to each other are determined.The combination of the delay plate 9 and of the splitter 10 a produces abeam splitter for splitting the incident light into right-circular andleft-circular polarised light. (Intensities I_(RZ) and I_(LZ) forright-circular and for left-circular polarised light). Hence fourdifferent polarisation components can be determined.

The four sought Stokes' parameters I, S, U and V can hence be determinedfrom the linear polarisation components (intensities I₀ and I₉₀) whichare detected by the receiving elements 11 a to 11 d, i.e. orthogonal toeach other, and from the circular polarisation components(right-circular polarised component with the intensity I_(RZ) andleft-circular polarised component with the intensity I_(LZ)) as follows

I=I ₀ +I ₉₀

S=I ₀ −I ₉₀

V=I _(RZ) −I _(LZ),

then with the secondary condition for monochromatic coherent laserradiation of

S+U+V=1

the fourth Stokes' parameter U=I₄₅−I₁₃₅ being able to be calculated.

The illustrated device for optical characterisation of FIG. 5 henceenables calculation of the complete polarisation state of the reflectedlight component Z from the received signal intensities of the fourreceiving elements 11 a to 11 d. Since the polarisation state of thereflected light Z depends upon the respectively examined sample materialof the sample P, the device shown in FIG. 5 can be used for materialcharacterisation of the sample P.

If receiver beam path and transmitter beam path are produced in the samehousing (integrated transmitting and receiving unit), a correspondingcharacterisation of the material can be effected provided that lightreflected on the sample (reflective) impinges on a retroreflector whichreflects the beams per se back to the combined transmitting andreceiving unit. In contrast to the arrangement with separate transmitterand receiver, the light is however reflected twice on the sample. Thepolarisation effects on the sample hence influence the receivedintensities quadratically.

What is claimed is:
 1. A device for characterizing a sample, the devicecomprising: a light source configured to illuminate the sample; adetector configured to receive light from the light source reflected bythe sample, capture a multi-pixel image of the sample from the receivedreflected light, and detect at least two different polarizationcomponents from the received reflected light; and a processor configuredto: determine a subset of image pixels of the captured image, such thatfor each image pixel in the subset of image pixels, the reflected lightcontributing to the image pixel is specularly reflected from the sample,and for each image pixel of the captured image not included in thesubset of image pixels, the reflected light contributing to the imagepixel is diffusely reflected from the sample, and for each image pixelin the subset of image pixels, output the detected values of the atleast two different polarization components.
 2. The device of claim 1,wherein: the sample defines a surface normal as being orthogonal to thesample at a center of the sample; the light source defines an incidentaxis as extending from a center of the light source to the center of thesample, the incident axis forming an incident angle with the surfacenormal; the detector defines an exiting axis as extending from a centerof the detector to the center of the sample, the exiting axis forming anexiting angle with the surface normal; and the processor determines thesubset of image pixels, such that for each image pixel in the subset ofimage pixels, the incident angle equals the exiting angle.
 3. The deviceof claim 2, wherein the incident angle equals a Brewster angle of thesample.
 4. The device of claim 2, wherein the light source includes aplurality of individual illumination elements positioned to illuminatethe sample with incident light from different directions.
 5. The deviceof claim 4, wherein the plurality of individual illumination elementsincludes two individual illumination elements.
 6. The device of claim 4,wherein the plurality of individual illumination elements includes fourindividual illumination elements.
 7. The device of claim 4, wherein theplurality of individual illumination elements are positioned in a planethat is orthogonal to the incident axis.
 8. The device of claim 7,wherein the individual illumination elements are spaced apart equallyalong a circle, wherein the incident axis intersects the plane at thecenter of the circle.
 9. The device of claim 7, wherein at least oneillumination element of the plurality of illumination elements ispositioned at an intersection of the incident axis and the plane. 10.The device of claim 1, wherein the light source comprises an individualillumination element.
 11. The device of claim 1, wherein the detectorincludes two cameras and a polarizing beam splitter, the polarizing beamsplitter configured to direct light having a first polarization state toone of the two cameras, the polarizing beam splitter configured todirect light having a second polarization state, orthogonal to the firstpolarization state, to the other of the two cameras.
 12. The device ofclaim 1, wherein two of the at least two different polarizationcomponents in the received reflected light are orthogonal to each other.13. The device of claim 1, wherein: the light source includes a laserconfigured to linearly scan the sample; and the detector includes areceiver configured to receive laser light reflected by the sample, thedetector further including a plurality of polarization-sensitiveelements for separating the received laser light according to thedifferent polarization states, the detector further including aplurality of detecting elements configured to receive the separatedlight in a one-to-one correspondence from the plurality ofpolarization-sensitive elements.